The present invention is directed toward optical communications, and more particularly toward a bulk optical grating multiplexer/demultiplexer having a flat-topped filter response and a grating for producing a flat-topped filter response.
At the inception of fiber optic communications, typically a fiber was used to carry a single channel of data at a single wavelength. Dense wavelength division multiplexing (DWDM) enables multiple channels at distinct wavelengths within a given wavelength band to be sent over a single mode fiber, thus greatly expanding the volume of data that can be transmitted per optical fiber. The wavelength of each channel is selected so that the channels do not interfere with each other and the transmission losses to the fiber are minimized. Typical DWDM allows up to 40 channels to be simultaneously transmitted by a fiber.
DWDM requires two conceptually symmetric devices: a multiplexer and a demultiplexer. A multiplexer takes multiple beams or channels of light, each at a discrete wavelength and from a discrete source and combines the channels into a single multi-channel or polychromatic beam. The input typically is a linear array of waveguides such as a linear array of optical fibers, a linear array of laser diodes or some other optical source. The output is typically a single waveguide such as an optical fiber. A demultiplexer spacially separates a polychromatic beam into separate channels according to wavelength. Input is typically a single input fiber and the output is typically a linear array of waveguides such as optical fibers or a linear array of photodetectors.
In order to meet the requirements of DWDM, multiplexers and demultiplexers require certain inherent features. First, dispersive devices must be able to provide for a high angular dispersion of closely spaced channels so that individual channels can be separated over relatively short distances sufficiently to couple with a linear array of outputs such as output fibers. Furthermore, the multiplexer/demultiplexer must be able to accommodate channels over a free spectral range commensurate with fiber optic communications bandwidth. Moreover, the devices must provide high resolution to minimize crosstalk and must further be highly efficient to minimize signal loss. In addition, a single device is preferably reversible so it can function as both a multiplexer and a demultiplexer (hereinafter, a xe2x80x9c(de)multiplexerxe2x80x9d). The ideal device would also be small, durable, inexpensive and scalable.
Diffraction grating based (de)multiplexers have significant advantages over other technologies for dense wavelength division multiplexing applications because of their relatively low cost, high yield, low insertion loss crosstalk, uniformity of loss as well as their ability to multiplex a large number of channels concurrently. However, grating-based (de)multiplexers typically have a Gaussian filter function. For long-haul fiber networks with large numbers of (de)multiplexers cascaded in series, a significant overall narrowing of the filter function occurs, ultimately leading to large insertion loss at the pass band edges. For smaller metro networks, it is not necessary to cascade large numbers of (de)multiplexers in series. However, deployment of metro network equipment is extremely cost sensitive, and a Gaussian filter function requires that the wavelength of the emitting lasers be locked to a particular wavelength with tight precision. But lasers tend to drift for a number of reasons, including variation in ambient temperature and aging and providing improved lasers adds significant cost to the network equipment. A flat-topped filter response places much less stringent requirements on the stability tolerance of the laser wavelength.
A number of alternatives have been proposed for adapting grating based (de)multiplexers to provide a more flat-topped filter function. One solution, used with planar waveguide arrays, is the used of a flares or parabolic waveguide input. Such structures are shown in Okawa, U.S. Pat. No. 6,069,990, and Dragone, U.S. Pat. No. 5,002,350. A similar solution has been taught for (de)multiplexers using bulk optical gratings. Finegan, U.K. Patent No. GB 2,219,869, teaches a waveguide coupling device having an array of first optical waveguides for carrying optical channels with different wavelengths and a second optical waveguide for carrying a wavelength division multiplex of the optical channels. A diffraction grating is provided between the waveguides to couple channels between the respective first and second waveguides. Each waveguide is provided with an expanded tapered core which effectively widens or broadens the filter function of the (de)multiplexer. Finegan teaches that the fiber core and surrounding cladding may be made of silica with the cladding region doped with fluorine or the core region doped with Ge. Heating of the fiber can cause dopant diffusion providing a tapered core having a fluted cross section. However, providing uniform heating to the fibers to yield consistent diffusion with high yields and at reasonable costs has proven elusive.
Another method for approximating a flat-topped filter response in a bulk optic diffraction grating is taught by Martin, U.S. Pat. No. 6,084,695. Martin teaches a (de)multiplexer structure having a planar array of single channel fibers. A converging lens array is located in an input plane optically coupled to the single channel fibers with the single channel fibers placed at the focal point of the lenses. Martin teaches that the use of the converging lens array effectively broadens the filter function, improving the tolerance of the system to variations in the pass bands. The use of the microlens array taught by Martin increases part count and therefore part costs and assembly complexity, and does not, by itself, adequately provide a flat-topped filter response.
Yet another way to provide a flat-topped filter response for a (de)multiplexer is taught by Lee, U.S. Pat. No. 5,999,290. Lee teaches the use of a 1 by 2 power splitter on an input waveguide and a 2 by 1 power splitter on an output waveguide to produce a flat-topped transmission band. Lee shows the power splitter used in conjunction with an arrayed waveguide (d e)multiplexer. Power splitters are known to introduce undesirable losses in the system.
Amersfoort, U.S. Pat. No. 5,629,992, discloses the use of a multimode interference (xe2x80x9cMMIxe2x80x9d) filter coupled to the end of a multi-channel fiber or single channel fibers in a grating based demultiplexer, respectively. The multimode interference filter is sized to multiply a singly peaked profile to effectively present a flattened top profile to thereby reduce sensitivity to wavelength drift. Use of the MMI filter prevents the apparatus taught in Amersfoort from being usable as both a multiplexer and a demultiplexer.
Trouchet, U.S. Pat. No. 6,141,152, discloses a (de)multiplexer which includes a compound focusing mechanism having adjacent focal points to produce multiple images for each wavelength signal. The resulting spectral response is said to be flattened in the vicinity of the center wavelength of each signal. The optical components that are required to achieve this compound focusing in free space are difficult to fabricate economically. Furthermore, the tolerances of the mechanical mounting are quite stringent, making the system impractical to produce.
Co-pending and commonly-assigned U.S. patent application Ser. No. 09/675,276, by Sappey, teaches a (de)multiplexer which provides a flat-topped filter response without requiring addition of optical elements which can increase the complexity of manufacturing and cost as well as degrade product efficiency. More specifically, Sappey discloses an apparatus for radially expanding the effective size of a waveguide receiving/transmitting end in combination with a diffraction grating optically coupled between a multiplex optical waveguide and single channel optical waveguides for diffracting an optical signal between the receiving/transmitting ends of the single channel optical waveguides. The diffraction grating has at least two surfaces optically coupled to the waveguides, each having a plurality of grooves therein. Each of the surfaces is angularly displaced relative to each other surface by a select amount such that a portion of the optical signal diffracted by each surface is offset in the direction of dispersion relative to the portions of the optical signal diffracted by the other surfaces to broaden the transmission band at the receiving/transmitting ends of the single channel and multiplex optical waveguides. However, it has been found that the apparatus of Sappey may have some resolution drawbacks.
The present invention is intended to provide a flat-topped filter response for (de)multiplexers while overcoming some of the problems discussed above.
One aspect of the present invention is an apparatus for use in optical communications systems to multiplex or demultiplex an optical signal comprising optical channel(s) of different wavelength(s). The apparatus includes a multiplex optical waveguide propagating a plurality of optical channels, with the multiplex optical waveguide having a receiving/transmitting end. The apparatus further includes a plurality of single channel optical waveguides, each propagating a single channel and each having a receiving/transmitting end. A diffraction grating is optically coupled between the multiplex optical waveguide and the single channel optical waveguides for diffracting an optical signal between the receiving/transmitting ends of the single channel optical waveguides. The diffraction grating has a plurality of grooves thereon, the spacing of which varied in accordance with an algorithm.
Another aspect of the present invention includes a method for designing and fabricating a grating having a plurality of grooves thereon, the spacing of which varies in accordance with an algorithm in order to produce a flat-topped response.
A (de)multiplexer made in accordance with the present invention provides a flat-topped filter response without requiring the addition of optical elements to the (de)multiplexer which tend to increase the complexity of manufacturing and cost as well as degrade product efficiency.